Mount Index, Mountain summit in King County, United States.

Mount Index is a peak in the central Cascade Range of King County, Washington, just south of the Skykomish River and U.S. Route 2. It consists of three pointed spires rising steeply from a low base; the main peak is the southernmost, while the North Peak rises more than 4,250 feet above the valley floor in less than one horizontal mile.

The mountain's composition formed more than 140 million years ago as igneous rock on the ocean floor. These ancient oceanic origins were later pushed to the surface through tectonic activity.

The name once belonged to neighbouring Baring Mountain; both peaks were renamed in 1917, leaving Mount Index as the former West Index Mountain. Climbers today approach the main peak from Lake Serene, while the North Peak's North Face Route, first climbed in 1929 by Lionel Chute and Victor Kaartinen, is a Grade III, Class 5.6 climb.

Reaching the summit requires technical climbing skills, proper equipment, and good route-finding through several options. The terrain is steep and exposed, so visitors should have alpine experience.

The granite here contains specific mineral compositions that make it particularly suitable for climbing. The crystal structure and surface texture of the stone create excellent conditions for rock climbers.